tools/nolibc: s390: disable stackprotector in _start

s390 does not support the "global" stack protector mode that is
implemented in nolibc.

Now that nolibc detects if stack protectors are enabled at runtime it
could happen that a future compiler does indeed use global mode on
and nolibc would compile but segfault at runtime.

To avoid this hypothetic case and to align s390 with the other
architectures disable stack protectors when compiling _start().

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>